[Pazuzu]

If you can make sure that you never lose pressure fully, then you can keep charging with R134 whenever you want. It'll slowly get worse and worse, but it's the cheapest quickest option to keep a system like that limping along.

If you ever lose pressure fully, then air (and moisture) will seep in, and you won't ever get it working well again without a proper overhaul. The reason you need pressure is even 20 or 30 psi of Freon in the system will prevent any air and/or moisture from getting IN, since there will be a positive pressure pushing OUT at any leak.

Now, this is all an order of magnitude solution. You will slowly get moisture working on, and the system will slowly get worse, and you will eventually not keep up enough and the pressure will drop too far. Get one of those top off cans of R134 with the gauge on it, and use it twice a season, you'll be cheaper than an overhaul for several years.